The present invention relates to a light-guide plate and a method for manufacturing the same, for example, to so-called front light which is set at display front side of various kinds of reflective type display units and the like including a reflective type liquid crystal to illuminate.
A back light and a front light are proposed for illuminating a liquid crystal display board and the like. In these units, light is guided to a light-guide member by providing a light source at a side face of the light-guide member comprising a transparent board, and plane light emission is realized by diffusing the light by using a light emission pattern such as prism grooves and the like provided at the surface of the light-guide member. Such the light-guide member is called a light-guide plate because it is generally a flat shape.
A method for forming a light emission pattern on the light-guide member by injection molding method for example is known as a technique forming such the light emission pattern. However, there are problems in the technique that shape fault of the light emission caused by lack of filling up resin material generates and it has a limit to make the light-guide member thin.
The technique forming the light emission pattern by performing press working to material of the light-guide member is proposed in JP-11-147255 for example. The technique using the press working has effects reducing the shape fault and design thin light-guide member.
However, making the light-guide member thin by the light emission pattern using press working has problems that thickness of an incident part of light source becomes thin and usage efficiency decreases.
Recently, a front light is becoming the main current, which illuminates display plane of liquid crystal directly by arranging at display front side of a reflective type liquid crystal instead of the back light illuminating the transparent type liquid crystal from back side. A display unit constructed by the reflective type liquid crystal and the front light has advantages that angle of visibility is made large and power consumption is made little.
Constructions are known as the front light, which are disclosed in Japanese Patent No. 2,925,530, JP-A-2001-110223, and JP-A-2001-23424.
In the construction of each of the above official gazettes, a face facing a liquid crystal of light-guide plate is made flat, half-circular concave portions or convex portions of a sawlike prism shape or a triangle prism shape in section are formed at face of the opposite side, and incident light from the end face of the light-guide plate is totally reflected at these prism-shaped face. By these constructions, illumination is performed by letting the liquid crystal direct, and uniform illumination over whole plane is performed by radiating from light emission face. Further, the construction bonding with liquid crystal through a refraction layer having smaller refractive index than the light-guide plate is disclosed in JP-A-2000-221501.
On the other hand, it is known that a light-guide plate for a front light is molded integratedly by injection molding method using light permeability resin material such as polycarbonate, methyl polymethacrylate resin, and so on. The process thereof will be described simply referring the flowchart of FIG. 11A, first, size of an external form of a light-guide plate determined based on the product specification and known values of quantity of light source and directivity are inputted to a simulation program of a personal computer so as to design for trial computing ideal arrangement of prism grooves and numbers of arrangement in Step S10. Continuously, the light-guide plate is actually produced by hand-made based on the design for trial in Step S11. Then, it is judged whether density of quantity of light from light emission face is uniform or not by setting a light source on the trial piece in Step S12, and when it is uniform, an exclusive injection-molding die is produced advancing the next step S13.
On the other hand, when density of quantity of light from light emission face of the light-guide plate is not uniform in Step S12, the light-guide plate as produced for trial again returning to Step S11 and repeats Steps S11 and S12 till density of quantity of light becomes uniform.
Next to the above Step S13, molding using the produced injection-molding die is performed so as to finish molding of the light-guide plate advancing to Step S14. Continuously, setting the light source in Step S15, whether density of quantity of light from light emission face of the light-guide plate is uniform or not, when it is uniform, it is shipped as a product advancing to the next Step S17.
On the other hand, when it is decided that density of quantity of light from light emission face of the light-guide plate is not uniform in Step S15, correction of the injection-molding die is performed advancing to Step S16, and the light-guide plate is molded again returning to Step S14 and repeats Steps S15, S16 and S14 till density of quantity of light becomes uniform.
It is indispensable condition that the prism grooves 7 are molded exactly in valley shape having planes to perform reflection of light as shown in FIG. 11B for the light-guide plate 1 molded integratedly with the prism grooves 7 by going through the above processes.
However, according to resin injection molding method using the above injection molding die, the prism grooves 7 are possibly molded in valley shape having dull planes as shown in FIG. 11C depending on the shape of the prism grooves 7, a part of molding, and condition of injection molding. When the prism grooves 7 are molded integratedly in such the valley shape having dull planes, it can not be used as a light-guide plate 1.
Further, it is possibly difficult to mold the light-guide plate integratedly since deformation such as bending and the like caused by heat contraction after injection molding generates. Further more, there are defects that an injection-molding die requiring accuracy of micron order such a light-guide plate requires is considerably expensive and life of the die is short. Additionally, since the correction of die for the above injection molding needs considerable skill, there is a problem needing man-hour for the correction.